1. Field of the Invention
The present invention relates to nuclear magnetic resonance equipment and process of using the equipment to produce magnetic spin echoes. It also relates to directional ultrasonic detection equipment.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of Related Art
Sonography has been used as an independent comparison system in (MRI. (See "ECG-Triggered Magnetic Resonance Tomographic Measurement of Blood Flow Velocity in the Cartid Arteries: Comparison with Duplex Sonography" by M. Seiderer, K. Kroner, E. Muller, F. Spengel in Digitale-Bilddiagn, September 1988, pp. 110-114.). An ultrasonic wave or light beam system has also been used to adjust automatically the magnetic gradient for the size of an object to be examined by (MRI. (See U. S. Pat. No. 4,558,425, entitled "NMR Imaging Apparatus of Changeable Inspecting Zone Size", by Yamamoto-Etsuji et al. ). Another invention uses ultrasonics to find, localize and visualize the object to be examined by (MRI. (See "U. S. Pat. No. 4,543,959, entitled DIAGNOSIS APPARATUS AND THE DETERMINATION OF TISSUE STRUCTURE AND QUALITY, issued Oct. 1, 1985 to Sepponen.). In these cases the usual ultrasonic pulse ranging and pointing procedures were used to help MRI equipment fix on a region of interest, but then the MRI equipment proceeded with its usual series of transmitter, field, and receiver pulsing sequences. (See "Ultrasound: Basic Principles", by R. Price, T. Jones, A. Fleischer and A. E. James, Jr., Chapters 12 and 13 in The Physical Basis of Medical Imaging, by C. M. Coulam et al., ed. Appleton-Century-Crofts, 1981. See also "The MRI Manual", Ch. 1, Yearbook Medical Publishers, 1990.). Position along one axis is encoded by measuring phase change which occurs during an interval when a spatial gradient field is pulsed on. Position along an orthogonal axis is measured by the precession frequency at a later interval when another gradient is pulsed on during reception of the spin echoes. In order to get reasonable accuracy, a variety of delay intervals are used in the phase measurement. This not only results in long total diagnostic time;, but signal decay proceeds during the delay due to irreversible, statistical losses of time constant T2. (See "The Origins and Future of Nuclear Magnetic Resonance Imaging", by F. W. Wehrli, Physics Today 45.). This decreases the signal-to-noise ratio. Since signal-to-noise ratio is directly proportional to magnetic field strength, the trend has been to higher fields. This builds in expense growth well beyond normal medical cost inflation. What is needed, is a system to locate and measure the spin resonance echoes without having to pulse magnetic gradients in high overall fields.
Nuclear spins interact with sound waves at twice the Larmor precession frequency as is shown by observed Raman Scattering. (See also, "Physical Electronics", by D. I. Bolef, Academic Press, vol. 4, 1965 and "Proceeding of I.E.E.E.", by D. I. Bolef and R. R. Sundfors, vol. 53, p. 1574, 1975 and "Soviet Physics Acoustics", by V. V. Shutilov, vol. 8 p. 303, 1963.). Application of a 25.33 MHz ultrasonic wave to KMnF.sub.3 caused transitions in the F.sup.19 nuclear magnetic resonance spectral line. (See "Physics Review Letters", by A Dennison, et al, vol. 12, p. 244, 1964.). Interferometry of optical signals, reflected from compressed wave-guides have been used to enhance sensitivity to ultrasonic signals. (See U. S. Pat. No. 4,959,539, entitled FLEXURAL DISK FIBER OPTIC HYDROPHONE, issued Sept. 25, 1990 to Hofler et al.).